FERGUSON, Mo., Aug 17 (Reuters) - Police arrested seven protesters in Ferguson, Missouri, early on Sunday as they imposed a curfew aimed at quelling days of violence that erupted after an unarmed black teenager was shot dead by a white police officer.

The latest confrontation occurred when demonstrators remained in the streets of the St. Louis suburb after the curfew took effect at midnight (0500 GMT). Seven people were arrested for failing to disperse, police said.

A person was shot and critically wounded during the night. It was not clear why, and the shooter was still at large, police said.

Missouri Governor Jay Nixon imposed the curfew on Saturday after a week of racially charged protests and looting over the Aug. 9 shooting of 18-year-old Michael Brown by Ferguson police officer Darren Wilson.

The Ferguson police department has come under strong criticism for both the shooting and its handling of its aftermath. On Sunday U.S. Attorney General Eric Holder ordered a federal medical examiner to perform an autopsy, in addition to one being conducted by state medical examiners.

Holder called for the federal autopsy "due to the extraordinary circumstances involved in this case and at the request of the Brown family," Justice Department spokesman Brian Fallon said. The family is also planning to have a pathologist conduct an independent examination of the body, a family spokesman said.

Unrest in Ferguson flared anew on Friday after the local police released a video showing the alleged participation by Brown in a convenience store robbery shortly before the fatal shooting. Police have said the officer who shot Brown had no idea he was a robbery suspect.

Nixon slammed the decision to release the video.

"I think it had an incendiary effect," he said on CBS' "Face the Nation," adding police "clearly are attempting to besmirch a victim of a shooting."

IMPOSING CURFEW

As the curfew took effect on Saturday night, law enforcement officials used loudspeakers to tell protesters to disperse. Officers, equipped with gas masks and full-length shields, stood among and on top of armored vehicles.

The person shot at a restaurant during the night was in critical condition, Missouri State Highway Patrol Captain Ron Johnson said. Police were unable to identify the victim, who he said was not shot by police.

Johnson, entrusted with restoring order by the governor, said canisters of smoke and later teargas were fired as part of police attempts to reach the victim of the shooting, "and not in relation to the curfew." The wounded person was taken to hospital by bystanders before police could reach him.

Johnson also said someone had shot at a passing police car but was not apprehended.

The smoke and teargas canisters largely dispersed the crowd, some of whom had been chanting "No justice, no curfew, no peace", while others implored the crowd not to move forward towards police.

'LEAST AMOUNT OF FORCE'

Nixon said that in spite of the clashes, the curfew was a success and the community deserved credit.

"We are always disappointed when things are not perfect, but thousands of people spoke last night, thousands of people marched and not a single gunshot (was) fired by members of the law enforcement," he told CNN's news show "State of the Union."

Nixon said he did not know how long the curfew would be in place. "We are trying to use the least amount of force to provide people the ability to speak while also protecting people's property," he said.

The clashes have pitted mostly black protesters against mostly white police in a residential and retail district.

Civil rights activist Al Sharpton, who was set to lead a rally with the dead teen's family on Sunday in Ferguson, called for an end to violence and looting.

"We cannot change it by becoming like those that we fight," he said on his syndicated radio show "Hour of Power."

"I say to the young people, I understand your anger and you have a right to be angry, but don't go mad and burn up your own community."

Brown's death has been described in markedly different ways by the police and by a friend who was walking with him at the time.

Police say that after Wilson asked Brown to move out of the road onto a sidewalk, Brown reached into the patrol car and struggled with Wilson for the officer's service gun. Wilson, who sustained a facial injury, then shot Brown a number of times.

The friend, Dorian Johnson, 22, and at least one other witness have said the officer reached out through his car window to grab at Brown and the teenager was trying to get away from the officer when he was shot. Brown held up his hands in a sign of surrender but the officer got out of his patrol car and shot Brown several times, they said.

The U.S. Department of Justice and the St. Louis County Police department are investigating. (Additional reporting by Lucia Mutikani and Julia Edwards in Washington; Writing by Colleen Jenkins; Editing by Chris Michaud, Alison Williams and Frances Kerry)

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